rockchip: adc: rockchip-saradc: factor out start_channel callback

SARADC v1 and v2 have a different way of starting a channel, therefore
let's abstract this function so that it can be provided from the
udevice.data pointer.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>